The inherited porphyrias are a family of disorders resulting from the deficient activity of specific enzymes in the heme biosynthetic pathway, also referred to herein as the porphyrin pathway. Deficiency in the enzymes of the porphyrin pathway leads to insufficient heme production and to an accumulation of porphyrin precursors and porphyrins, which are toxic to tissue in high concentrations.
Of the inherited porphyrias, acute intermittent porphyria (AIP, e.g., autosomal dominant AIP), variegate porphyria (VP, e.g., autosomal dominant VP), hereditary coproporphyria (copropophyria or HCP, e.g., autosomal dominant HCP), and 5′ aminolevulinic acid (also known as δ-aminolevulinic acid or ALA) dehydratase deficiency porphyria (ADP, e.g., autosomal recessive ADP) are classified as acute hepatic porphyrias and are manifested by acute neurological attacks that can be life threatening. The acute attacks are characterized by autonomic, peripheral, and central nervous symptoms, including severe abdominal pain, hypertension, tachycardias, constipation, motor weakness, paralysis, and seizures. If not treated properly, quadriplegia, respiratory impairment, and death may ensue. Various factors, including cytrochrome P450-inducing drugs, dieting, and hormonal changes can precipitate acute attacks by increasing the activity of hepatic 5′-aminolevulinic acid synthase 1 (ALAS1), the first and rate-limiting enzyme of the heme biosynthetic pathway. In the acute porphyrias, e.g., AIP, VP, HCP and ADP, the respective enzyme deficiencies result in hepatic production and accumulation of one or more substances (e.g., porphyrins and/or porphyrin precursors, e.g., ALA and/or PBG) that can be neurotoxic and can result in the occurrence of acute attacks. See, e.g., Balwani, M and Desnick, R. J., Blood, 120:4496-4504, 2012.
The current therapy for the acute neurologic attacks is the intravenous administration of hemin (Panhematin®, Lundbeck or Normosang®, Orphan Europe), which provides exogenous heme for the negative feedback inhibition of ALAS1, and thereby, decreases production of ALA and PBG. Hemin is used for the treatment during an acute attack and for prevention of attacks, particularly in women with the acute porphyrias who experience frequent attacks with the hormonal changes during their menstrual cycles. While patients generally respond well, its effect is slow, typically taking two to four days or longer to normalize urinary ALA and PBG concentrations towards normal levels. As the intravenous hemin is rapidly metabolized, three to four infusions are usually necessary to effectively treat or prevent an acute attack. In addition, repeated infusions may cause iron overload and phlebitis, which may compromise peripheral venous access. Although orthotrophic liver transplantation is curative, this procedure has significant morbidity and mortality and the availability of liver donors is limited. Therefore, an alternative therapeutic approach that is more effective, fast-acting, and safe is needed. It would be particularly advantageous if such treatment could be delivered by subcutaneous administration, as this would preclude the need for infusions and prolonged hospitalization.
AIP, also referred to as porphobilinogen deaminase (PBGD) deficiency, or hydroxymethylbilane synthase (HMBS) deficiency, is the most common of the acute hepatic prophyrias. It is an autosomal dominant disorder caused by mutations in the HMBS gene that result in reduced, e.g., half-normal activity of the enzyme. Previously, a mouse model of AIP that has ˜30% of wildtype HMBS activity was generated by homologous recombination. Like human patients, these mice increase hepatic ALAS1 activity and accumulate large quantities of plasma and urinary ALA and PBG when administered porphyrinogenic drugs, such as phenobarbital. Thus, they serve as an excellent model to evaluate the efficacy of novel therapeutics for the acute hepatic porphyrias.